The use of a viscous fluid clutch for controlling the rotation of an engine cooling fan for a vehicle is well known in the art. A multi-blade fan is removably secured to a viscous fan clutch installed between an accessory pulley (typically the water pump pulley) of a vehicle and a radiator. The viscous clutch is designed to drive the fan at high speeds which can approach the input speed of the viscous clutch which is normally the rotational speed of the engine. The operation of the viscous clutch is controlled based upon the engine's operating temperature to drive the fan approaching engine speeds when cooling is required and the viscous clutch permits the fan to operate at low speeds when cooling is not required. The thermostatic control of the fan through the clutch reduces airflow noise caused by fan rotation and load on the engine which provides the benefit of increased horsepower and improved fuel economy.
Generally, in the prior art clutches, a clutch plate housed within the clutch assembly, having lands and grooves, is mated to a housing having complimentary lands and grooves. An internally contained pump plate separates a working chamber from a reservoir. Gates in the pump plate permit selective flow of a viscous fluid from the reservoir to the working chamber and into a shear zone between the lands and grooves of the housing and the clutch plate. Fluid shear in the lands and grooves transfers input torque from the clutch plate to drive the housing and the attached fan.
When cooling is not required, gates in the pump plate are closed and the fluid in the shear zone is pumped into the working chamber. Orifices in the pump plate permit passage of the fluid from the working chamber to the reservoir. The removal of a majority of the fluid reduces the shear between the clutch plate and the housing, thereby substantially reducing the rotation of the housing and the attached fan.
The prior art viscous clutch systems which are thermostatically controlled rely on the temperature of the air passing through the radiator or the temperature of the cooling water of the engine to switch the on/off control for the fan. While these prior art viscous fan clutches have met with wide acceptance with the automotive and truck manufacturers, closer or more accurate control of the viscous fan clutches offer additional advantages to these manufacturers. Engine control management systems have been incorporated into most automotive and truck vehicles to control emissions, fuel economy as well as other engine operating characteristics. It would be advantageous to have a viscous fan clutch which could be controlled by the engine control management system and provide the ability to variably control the engagement of the clutch and thus the rotation of the fan.